An increased incidence of lymphoma in adult C3H/HeJ mice injected with antithymocytic serum as compared with normal rabbit serum during a course of one to four weekly intraperitoneal injections of cell-free filtrates of Gross virus-induced lymphomas was noted. The latent period of lymphoma ranged from 119 to 298 days after initiation of treatment. The incidence tended to increase as the number of injections of cell-free filtrate was increased, with a maximum incidence reached at three injections.